The present invention relates generally to securing a sheet medium onto a support surface and, more particularly, to a method and apparatus for supporting a flexible film sheet medium on a relieved support surface of a print drum which desensitizes the systems to the effects of contamination and air turbulence for allowing the sheet to be imprinted with improved resolution.
A wide variety of sheet processing systems has been proposed for effecting clamping of a sheet medium onto a cylindrical surface of a rotatable print drum. For example, in facsimile machines, computer printers, and xerographic copiers, there are requirements for releasably clamping and wrapping a sheet medium to and about a rotary drum, whereby the medium can be imprinted while the drum is rotating. In general, the rotary drums of the above devices are rotated at relatively slow speeds, for example, in the order of about 10-100 rpm. However, with the advent of high speed digital dry laser imaging processes, such as the type commercially available from Polaroid Corporation of Cambridge, Mass., USA for use in obtaining high-quality radiographic images, there is a requirement that the film or medium be printed while being rotated at high speeds, such as in the order of about 1200 to 6000 rpm in order to produce images within the commercially accepted time frame of conventional techniques.
Another requirement is that the sheet being imaged remain in a preferred wrapped position for insuring the degree of image resolution required in the medical field. For example, one consequence of a sheet being misaligned or spaced from its desired wrapped position is that the quality of the resolution can be compromised significantly. This is especially critical with, for instance, radiological images of the medical type. It is evident that even minor deviations of a sheet from its intended wrapped position may cause unacceptable medical images.
The potential for a sheet deviating from its intended wrapped condition during digital imaging of the above type becomes even more significant whenever the size of the sheet to be printed increases. This is so because the larger format film sheets must be rotated at higher speeds so that they can be imprinted with considerably more information within the same general time frame as the smaller format sheets having less information. Because of increases in rotational speeds, there are increases in centrifugal forces acting on the sheet. If the film sheet bulges or otherwise separates from the drum surface irregularly, then the printing laser head, which automatically moves toward and away from the sheet during printing in an effort to maintain the laser head at its desired focal plane distance to the print surface, will not be able to move or track in and out fast enough to maintain the desired focal distance. As a consequence, the rendered radiological images can be less than satisfactory. This tends to create problems with the sheet separating radially.
The problem with sheet profile deviation such as bulging may be further aggravated when dirt or other contamination is present on the drum surface or on the medium. The dirt or contamination may be in the form of minute dust particles which cause a localized disturbance in the medium. In particular, such dust particles can create so-called tenting issues, whereby the media is disturbed from a smooth profile such as being sloped or inclined from the dust particles. Such tenting in effect magnifies the disturbances to the desired media shape when wrapped. It also has been further determined that air turbulence over and under the sheet will cause the media shape profile to change undesirably.
One known approach for clamping a flexible sheet of dry laser imaging film onto a cylindrical surface of a rotatable drum, so as to be imprinted by a laser, is described in commonly assigned U.S. Pat. No. 4,903,957, issued Feb. 27, 1990. This patent discloses use of leading and trailing edge clamps which are mounted axially on a rotatable drum and are sequentially operated by external cams to clamp and release both the leading and trailing edges of the flexible sheet that is to be wrapped on a rotating drum. Efforts have been undertaken to maintain the media shape on the drum. In this regard, some of the clamps proposed tend to minimize the radial separation of the sheet by clamping the sheet so that it is tightly wrapped on portions of the drum surface. An example of such latter clamping is described in U.S. Pat. No. 5,516,096, which is incorporated herein as a part hereof.
While the foregoing approaches are satisfactory, there is nevertheless a desire to improve upon printing performance by reducing sensitivity of the media and thus the autofocus to dust and debris and air turbulence that causes displacement of the media from a desired shape, especially in situations where high rotational speeds are to be encountered.